In this field, a high-velocity, high-temperature flame is directed against the stone surface to produce spalls of individual as well as multiple grains.
Although oxy-fuel flames are often used for this purpose, their operating costs are high. Compressed air-fuel oil flames may also be used where the reactants are burned within an enclosed combustion chamber with the hot products of combustion expanding through a restricting nozzle to form a jet flame which is directed against the rock surface.
Such an air-fuel burner is described in my U.S. Pat. No. 3,211,242 issued July 23, 1963. Further, this patent discloses that maximum stone removal rates are associated with flame burners of the internal combustor type operating between 3 and 15 psig. Such pressures produce a high-velocity, but subsonic flame jet. Combustion at this low pressure level is difficult and requires large diameter burners to produce useful flows of hot gas.